This invention relates to an etching method for forming deep grooves in a semiconductor substrate by reactive ion etching.
In general, wet etching using an etching liquid has been widely used for producing semiconductor circuits and the like for many years. However, wet etching has many disadvantages such as the occurrence of undercutting. As the requirement of much finer circuit patterns increases with improved integrated degrees and the like, so-called dry etching methods without using any etching agent have been proposed. Among them, the reactive ion etching method has been of particularly note. This method uses parallel plate electrodes and accomplishes simultaneous use of physical etching such as sputtering or the like and chemical etching by a chemical reaction.
With this method, the parallel plate electrodes are arranged in a reaction vessel, and after a substrate to be etched has been located on one of the electrodes, an etching gas under a predetermined pressure is introduced into the reaction vessel. A predetermined high frequency electric power is then applied to the parallel plate electrodes in the etching gas atmosphere to produce a plasma in the reaction vessel. The resulting physical and chemical reactions etch the substrate.
In etching a silicon substrate by the reactive ion etching method of this kind in general, various gases are used for this purpose. In most cases, the gases are fluorinated hydrocarbon compounds such as CF.sub.4, CHF.sub.3, other hydrocarbon fluoride compounds and the like, gases containing chlorine, and gases containing bromine.
However, etching using a gaseous fluorine compound is likely to cause undercutting resulting in a problem in processing accuracy. On the other hand, etching using a chlorine-containing gas has less chance of side etching in comparison with that using a gaseous fluorine compound; however, there is a tendency in using a chlorine-containing gas for a surface of a silicon substrate to be etched into a rough surface which appears to be black. In etching using the bromine-containing gas, moreover, side walls of grooves formed by the etching often leave projections thereon to form rough surfaces.
Furthermore, in the respective etching methods using the above various gases, side walls of etched grooves tend to be curved, thus deviating from desired vertical surfaces, and sometimes fine grooves are unintentionally formed in bottoms of the etched grooves. Etching using silicon tetrachloride (SiCl.sub.4) and oxygen (O.sub.2) exhibits a high selectivity for silicon dioxide which is usually used as masks and achieves comparatively good etched configurations in comparison with the etching methods using the other gases. However, reaction products consisting of silicon and oxygen compounds formed in etching produce a white powder which contaminates the reaction vessel and the substrates. Moreover, the white powder acts as if it were a mask at locations to be etched, so that portions of a substrate to be etched remain unetched even after an etching step. A substrate is etched faster at a portion surrounding the white powder than at a portion in direct contact with the white powder, so that grooves are formed in bottoms of etched grooves or the bottoms are uneven or appear to be black. Furthermore, in etching using silicon tetrachloride, the etched configuration is likely to be detrimentally affected by residual gases in the reaction vessel, with the result that the etched configuration tends to be unstable.
With the above etching methods using the various kinds of gases, grooves not having undersized curved portions, i.e., "anisotropic etching" may be temporarily accomplished by properly adjusting various etching conditions such as the flow rate ratio (a ratio of gas to the total volumetric flow rate). However, setting the etching conditions is very delicate, so that even if the etching is effected with the same conditions, the etching is not reproducible.